Bryan Caplan  

Bill and Robin's Not So Excellent Hypothesis

Computer Models as Autocrats... David Kennedy talks to Russ Ro...
Robin explains a new theory of falling fertility that he cooked up with Bill Dickens on the road back from GenCon:
The key idea: farming pressures strengthened a fem forager tendency to, when personally richer, invest more energy in pursuing status, relative to raising kids. So when all fems are rich, they all invest more in status, relative to kids, and fertility falls.
Robin then explains the theory in great detail.  But unless I'm deeply misunderstanding it, this "excellent" theory doesn't even get off the ground.  Like Feyrer and Sacerdote's theory, Dickens-Hanson implies gender conflict: In the modern world, men should want more children than women, and this gap should get larger as people get richer.  But in reality, men and women around the world see eye-to-eye on this question - see the World Values Survey, question D017.

But doesn't the Dickens-Hanson mechanisms work for men, too?  Robin thinks it does, but admits that it doesn't work as strongly:
Much of this analysis also applies to men... [M]en today may also devote excess effort to developing status markers. The main differences seem to be that 1) men tend less to directly raise kids, 2) men can father kids at older ages, and 3) men gain more reproduction from being high status.  So men should work even harder to gain status markers. But even so, raising overt kids will less distract men from pursuing high status, and a man's delay in starting kids will less reduce his fertility.  Thus excess male status efforts probably do less to reduce overall fertility.
Do I have a better story of declining fertility?  Not one that's methodologically acceptable to Robin.  But there's one thing I do know - any theory that implies serious gender conflict is wrong.

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COMMENTS (9 to date)
Chris writes:

You should have at least titled the blog post, "Bill and Robin's Bogus Theory."

Hyena writes:

I think it's strange to assume that a species which has had both language and complex social organization as long as we have would have a very simple-to-reduce reproductive drive.

We have vast amounts of cultural norms surrounding sex, relationships and reproduction, many of them are geared towards encouraging the former two. Generally these get treated as if they were merely methods of social control; ways to manage a population or secure political power through lineage. Isn't at least somewhat plausible that the existence of these "cultural technologies" indicate that there are complex problems to be solved in human reproductive urges?

Chris T writes:

There is some evidence that fertility rates begin rising again past a certain level of development:

Pandaemoni writes:

What I have always wondered: if fertility rates are declining because we are pursuing a rational utility maximizing lifestyle, then aren't we modern westerners doomed by natural selection to lose out to a group that pursues overall resproductive success (success measured in a Darwinian sense) and not utility maximization?

One possible reason we may not be doomed is that our personal satisfaction may be entirely consistent with long term reproductive success. We may have fewer kids, for example, but perhaps they will receive better care and attention, thus conferring a survival advantage. That's possible, but my intuition is that there is a substantial difference between the two goals, and that many will quite consciously limit their own and their progeny's chances of long term reproductive success because they don't want the responsibility of parenting.

It seems as if—if a subspecies of homo sapiens that was single mindedly focused on the reproductive success of themselves and their progeny—my lazy and hedonistic group would be doomed. We may have the happier lives, but evolutiuon doesn't select for happiness.

Doc Merlin writes:

Religious and political conservativeness is a far better predictor for family size than Robin's theory. It also explains why rich republican families tend to be large, but rich democratic families tend to be small. Also, why mormon families tend to be large.

Tracy W writes:

Pandaemoni- the doom seems doubtful. For a start, Westerners vary in how many children they have. (For example, I know people who purposefully stopped with two children, others who purposefully had three, or four, or five). Plausibly, there may be some genetic component to the variation (what's the identical twin data on number of children?)

So over generations, those people who have more children would gradually displace those who have fewer children, for whatever genetic reasons, within Western population, resulting in a higher average birth rate for the Western population.

Of courae it might be that another, non-Western group, with a higher birth rate, would outbreed the Westerners first. Or it may be that conditions would change so much that those factors that made for reproductive success in western countries in the 20th and 21st centuries would not make for reproductive success in the future, so it would all be up in the air again. So there are at least two options other than "doomed".

Pandaemoni writes:

Tracy W:

It doesn't compeletely reduce to birth rate, of course. It's possible for a set of parents to have 10 children, but due to limited resources like time and food, raise them so poorly (in a certain Darwinian sense) that the reproductive chances of those children are materially lessened (as compared to, say, a family with 5 children).

In the long run, what should matter from an evolutionary perspective is which strategy is best geared towards reproductive success. Unlike most animals, humans can consciously decide what sort of strategy to adopt in living our lives. We tend to adopt a strategy that roughly approximates utility maximization.

I simply wonder if in the long run a strategy that consciously maximizes reproductive success would not be strictly much so that we utility maximizers might have no future. Even if the two coexist for a long time, survival of the fittest is a long game (we hope).

Peter Finch writes:


Your hypothetical family that has materially worse reproductive prospects with 10 children, but can afford 5, makes sense if we're talking about the developing world. But in the modern western world, how much income do you really need to insure 10 kids are reproductively fit? That doesn't require sending them to Harvard or buying them iPods.

I would guess that it's at or about the poverty line, and that what we think of as investing in our children, say, by reading to them, taking them to soccer, or going to parent-teacher night, really isn't doing much for their reproductive fitness. Once you feed them, don't beat them, and have two parents physically present, you probably have most of the Darwinian bases covered.

I'm (sort of) with TracyW on this: If family size is heritable (whether genetically, or in the same sense that values are heritable), birth rates are going to go back up. There's some sort of temporary shock going on.

Bill Drissel writes:

Kids are expensive. Kids keep ya from goin' to:
*water skiing
*all weekend drinking sessions

Kids need station wagons, not Corvettes.
and the beat goes on. Gotta really want 'em. Gotta really love 'em.

Bill Drissel

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